Now Envia Systems, a start-up based in Newark, Calif., has announced it has achieved a critical milestone: a rechargeable lithium-ion battery with an "energy density" of 400 watt-hours per kilogram, the highest energy density known to be recorded.

When commercialized, Envia says the 400 wh/kg battery, with a range of 300 miles and a cost of about $25,000, will slash the price of electric vehicles and make them more affordable for mainstream consumers.

Envia received a $4 million grant from ARPA-E in December 2009 to develop advanced lithium-ion batteries for electric cars. It went on to raise $17 million in venture capital from General Motors Ventures, Bay Partners, Redpoint and Pangaea Ventures.

"We hope that this low cost and high density battery technology enables widespread adoption of electric vehicles across the country and around the world," said ARPA-E (Advanced Research Projects Agency-Energy) Director Arun Majumdar.

Batteries are complex systems that convert stored chemical energy into electricity. Researchers say advances often involve trade-offs: improving range may result in skyrocketing costs, or a shorter battery life.

Measured as kilowatt hours per kilogram or liter, "energy density" determines range: The more watt hours, the more miles a car can travel on a single charge. Low-cost, high-energy density batteries are the Holy Grail. Battery costs are expected to come down due to volume manufacturing, but energy density has been a much harder goal to achieve.

Kapadia said that Envia's hard-working team of engineers developed the technology from scratch. After testing the battery in-house, additional testing was performed by the Electrochemical Power Systems Department at the Naval Surface Warfare Center in Indiana. The company is now in discussion with auto manufacturers.

The Tesla Roadster, Nissan Leaf and Chevrolet Volt all use some form of lithium-ion chemistry in their batteries.

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